Cassandra is a decentralized (fast reads), highly available (fast writes), fault tolerant database that can allow you to scale out well beyonds what's available with traditional RDBMS like MySQL.

Index optimization, database denormalization, replication and sharding are great techniques to squeeze more juice out of MySQL...

But eventually, as your tables and queries grow, you're going to hit a brick-wall.

Storing huge amounts of data with MySQL is easy. But when it comes time to Retrieve those records, using filters, sorts and joins, you'll be lucky if you can ever scale beyond 1 million records while still maintaining high front-end speeds.